Coalescing agents are generally used in paints, varnishes and water-based adhesives. Coalescing agents act as a solvent and/or a plasticizer to the polymeric phase of paints, varnishes and adhesive formulations, diminishing the Tg thereof and favoring the formation of continuous and homogeneous films at several temperature and humidity conditions. Coalescing agents need to be stable in the presence of water and compatible with ingredients present in paints, varnishes and adhesive formulations. Specially, coalescing agents cannot destabilize polymer particles present in paints, varnishes and adhesive formulations. Coalescing agents should also be sufficiently volatile to be eliminated during drying of films, preferably after water evaporation.
The plasticization power of a coalescing agent, that is, the ability thereof in diminishing the Tg of a polymer, depends on solubility parameters of the coalescent and the polymer. The solubility parameter is associated with the cohesive energy of solvents and solutes. According to the Hansen model, the cohesive energy comprises the sum of contributions from non-polar or dispersive (Delta D) interactions, polar interactions (Delta P) and hydrogen bonds (Delta H). Usually, solubility of a solute in a given solvent is maximized when the solubility parameters are the same or close to each other. This rationale follows the general rule that like dissolves like. The similarity degree between solvent and solute solubility parameters is provided by the radius of interaction, which takes into account the differences between contributions of dispersive and polar interactions and hydrogen bonds of the solute and the solvent. If the radius of interaction of the solute-solvent combination is lower than the radius of the solubility sphere of the solute, the solvent is likely to dissolve the solute. Usually, water-insoluble coalescing agents having solubility parameters that are compatible with those of the polymer to be plasticized are more effective in reducing MFFT than water-soluble coalescing agents.
Currently, there is an increasing need for coalescing agents that meet the environmental legislation that controls emission of volatile organic compounds (VOC) to the atmosphere. According to European Directive legislation 2004/42/CE coalescing agents having boiling point of less than 250° C. are considered volatile organic compounds. The Green Seal Guidelines recommends using coalescing agents having boiling point greater than 280° C. Other features of coalescing agents that are valuable to manufacturers of paints, adhesives and varnishes and for the final consumer are: low odour, HAP-free, renewable raw material-derived coalescents having high vegetalization index and low CO2 emission during the manufacture process. Table 1 below presents the VOC levels recommended by the European Directive Legislation 2004/42/CE for paints and varnishes.
TABLE 1VOC levels recommended by the European Directive Legislation2004/42/CE.VOCVOC thresholdthreshold (g/L)(g/L)Product subcategoryJan. 01, 2007Jan. 01, 2010Matt coatings for interior walls and7530ceilingsGlossy coatings for interior walls150100and ceilingsCoatings for exterior walls of7540mineral substrateInterior/exterior trim and cladding150130paints for wood, metal or plasticInterior/exterior trim varnishes and150130woodstains including opaquewoodstainsInterior and exterior minimal build150130woodstainsPrimers5030Binding primers5030One-pack performance coatings140140Two-pack performance products140140for a specific final use, namely,floorsMulti-coloured coatings150100Decorative effect coatings300200
Several molecules having alcohol and/or ester functionality are used as coalescing agents. Butyl glycol and 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (Texanol from Eastman Kodak) are the molecules most used as a coalescent. Due to its boiling point of 169-173° C. butyl glycol is considered a VOC by the European Legislation. Texanol, which has a boiling point of around 254° C. is not considered a VOC by the European Legislation. Nevertheless, Texanol does not meet the Green Seal Guidelines, which recommends that products used in civil construction be formulated with coalescing agents having a boiling point greater than 280° C.
Patent literature contemplates several reports mentioning the use of esters as coalescing agents.
US patent application US2010/0130645A1 to Elementis Specialties, entitled “VOC-Free Coalescing Agent”, describes the use of monoesters of ethoxylated and/or propoxylated fatty acids as coalescing agents. The monoesters of the invention have structural formula R1COO[(EO)x(PO)y]zH. The R group contains from 6 to 22 carbon atoms, is linear and saturated, x and y may range from 0 to 5 and z ranges from 1 to 5.
The main esters are:                Monoesters obtained from fatty acids and ethylene glycol or propylene glycol        Esters obtained from fatty acids and ethene oxide and/or propene oxide        Formulations containing at least 70% propylene glycol monolaurate        
Results of performance evaluation have shown that propylene monolaurate, which has a boiling point of >265° C., decreases MFFT (minimum film-forming temperature) of Rhoplex® ML200™ latex, MFFT of 6° C., with the same efficiency as Texanol coalescent.
U.S. Pat. No. 4,489,188 to Eastman Kodak Company, entitled “Coalescent-Containing Coating Composition”, describes the use of monoesters obtained from the esterification of benzoic acid-derived acids and ethylene glycol and/or propylene glycol-derived ethers. The esters of the present invention were shown to be more effective in reducing MFFT of several styrene-acrylic latexes than Texanol.
US patent application US20070/0093579 to Curators of the University of Missouri, entitled “Water Born Film Forming Compositions”, describes the use of esters of fatty acids derived from vegetable oils and methanol or glycol derivatives. The fatty acids used in esterification reactions are from corn, sunflower, soy and flaxseed oils. The esters thus obtained have hydrocarbon chains with at least two unsaturations. Glycol esters of fatty acids present in soy oil reduced MFFT of UCAR 430 latex in a similar manner as Texanol.
International Patent Application WO2008054992 to Velsicol Chemical Corporation, entitled “Aqueous Film Forming Compositions Containing Reduced Levels of Volatile Organic Compounds”, describes the use of mixtures of benzoic acid monoesters and diesters and ethylene glycol or propylene glycol derivatives as coalescing agents. Paint films from formulations containing mixtures of diethylene glycol dibenzoate, dipropylene glycol dibenzoate and monobenzoates have greater wet abrasion resistance than films from paint formulations containing Texanol.
US patent application US2009/0198002 A1 to Rhodia INC entitled “Low VOC Coalescing Agents”, describes the use of diesters of adipic, glutaric and succinic acids and alcohols comprising a hydrocarbon chain having 4 to 8 carbon atoms, including alcohols from fusel oil. Diesters obtained from glycol ethers and diacids are also encompassed in the invention. Diesters derived from glutaric and succinic acid and butanol or pentanol were more effective in reducing MFFT than Texanol.
Based on the above-mentioned examples, one notes that the use of esters as coalescing agents is extensively explored and among these esters the following are important: monoesters of ethoxylated and propoxylated acids, methyl and glycol esters of vegetable oil fatty acids, monoesters of benzoic acid and glycols, a mixture of monoesters and diesters of benzoic acid, glycols and diesters from diacids. Nevertheless, mixtures of monoesters and diesters originating from the ethoxylation of fatty acids and/or esterification of glycols and fatty acids have not been contemplated so far.
In view of that, it is evident that the field of the present invention, i.e., the use of mixtures of monoesters and diesters as coalescing agents, can still be developed.